Add -pthread for FreeBSD build procedure.

Required by boost library.
Add FreeBSD build environment settings.
Add FreeBSD into configure host list.

Tested on FreeBSD 9 with following steps:

 # For dependency
 pkg_add -r wget protobuf protobuf-c v8 openjdk7 boost-all gmake
 pkg_add -r bash node npm py-yaml gcc48
 npm install -g less coffee-script handlebars

 # To configure
 CXX=gcc48 ./configure --ccache
 # Edit config.mk to add "ALLOW_WARNINGS := 1"

 # To build
 gmake